To applicant's knowledge, the use of relative translational movement in butterfly valves as described above was first set forth in the above-referenced application, Ser. No. 374,897. The importance of such movement is that it is accompanied by a change in the effective areas of the valve plate on each side of the pivot axis or, put differently, by a change in alignment of the pivot axis with the center of pressure exerted by fluid on the valve plate. This enables adjustment of the net torque exerted on the valve plate by the fluid conveyed through the valve. The net torque can be decreased to stabilize the valve plate or to passively facilitate rotation thereof, and it can be increased to actively rotate the plate or facilitate such rotation.
In prior arrangements for effecting the translational movement, the general approach has been to provide a valve plate which is translatable relative to a pivot shaft which has a stationary longitudinal axis. The above-referenced applications, Ser. Nos. 374,897 and 422,354, illustrate arrangements in which the pivot shaft is translationally movable relative to the valve plate when the latter is in a closed position. That movement is provided to offset the pivot axis from the center of static fluid pressure in order to effect initial rotation of the valve plate away from the closed position. Subsequent translational movement for control purposes occurs as movement of the valve plate relative to the pivot shaft. Although it is ostensibly recognized in application Ser. No. 374,897 that translational movement can be provided for control purposes via a pivot shaft which is movable relative to the valve plate, no associated structure has been described.